The overall goal of the research studies proposed here is to obtain high-resolution structures of intact hetero- multimeric N-methyl-D-aspartate receptors (NMDARs). NMDARs belong to the family of ionotropic glutamate receptors, which mediate the majority of excitatory synaptic transmission in mammalian brains. Dysfunctional NMDARs are implicated in various neurological disorders and diseases including schizophrenia, depression, Alzheimer's disease, and Parkinson's disease. A unique aspect of NMDARs is that they are obligatory hetero- tetramers or higher oligomers composed of GluN1 and GluN2 (A-D) or GluN3 (A-B) subunits. Opening of NMDAR ion channels requires binding of glycine to GluN1 and GluN3 and glutamate to GluN2. To date, structural studies of NMDARs have been limited to the hetero-dimeric structures of the GluN1 and GluN2 extracellular domains. Thus, there is no clear knowledge on how subunits and domains are arranged to form hetero-multimeric ion channels and how transmembrane ion channel pores are shaped to confer specific properties of NMDAR ion channels including high calcium conductance and voltage-dependent magnesium block. Despite various technological breakthroughs, success in crystallographic studies on eukaryotic membrane proteins has been limited due to difficulties in expression, purification, and crystallization stemming from sample heterogeneity and instability. Importantly, there has been no crystal structure of eukaryotic hetero- multimeric membrane proteins that are recombinantly produced to date. The fact that numerous ion channels, G protein-coupled receptors, receptor kinases, and intramembrane proteases implicated in neurological diseases exist as hetero-multimers in native states points to the great need for structural studies on hetero- multimeric membrane proteins. To obtain the first crystal structure of hetero-multimeric ion channels and to understand the structure-function relationship of NMDARs, we will conduct research with the following two aims: Aim 1 is to produce intact hetero-multimeric NMDAR proteins using our novel methodology and to biochemical characterize the homogeneously purified proteins; and Aim 2 is to complete structural analysis of intact NMDARs in complex with various ligands reflecting different functional states by applying cutting-edge techniques in membrane protein crystallography and validate structure-based functional hypotheses by biochemical and electrophysiological experiments. Successful completion of the proposed studies is expected to result in the first crystal structure of a hetero-multimeric ion channel and to provide a mechanistic understanding of NMDARs that are critical in brain physiology and development. Importantly, the structural information obtained here will also provide strategies to develop compounds with therapeutic efficacy in neurological disorders and diseases. Furthermore, these studies on NMDARs will establish fundamental guidelines for crystallography on hetero-multimeric membrane proteins.